How it works: Positron Emission - University of Washingtoncourses.washington.edu/bioen508/Lecture5-B-PET.pdf · How it works: Positron Emission Radioactive decay •unstable atomic

How it works: Positron Emission

Radioactive decay• unstable atomic nuclei due to too

many protons relative to thenumber of neutrons

• decays to stable form byconverting a proton to a neutron

• ejects a 'positron' to conserveelectric charge

• positron annihilates with anelectron, releasing two anti-colinear high-energy photons

npnp

n

pnp n

pn

pn p

p

pn

p n

pn

p

n

p n npnp

n

pnp n

pn

pn p

n

pn

p n

pn

p

n

p n

~2 mm

18F 18O

~180 deg

E = mc2

= 511 keV

β+

e-

Types of Photons

• X-ray photons• gamma (γ) ray photons (Greek letters for

radiation from nuclear decay processes)• annihilation photons

all can have the same energy

Molecular Imaging: Glu Metabolism

FDG-6-PO4 is ‘trapped’ and is agood marker for glucosemetabolic rates*

glucose

glucose 6-phosphate

pyruvate lactate

gylcolysis(anaerobic,inefficient)

TCA(oxidative,efficient)

HOCH2

H 18F

HOH HHO

H

OH

H

radioactivefluorine

O

[18F]fluorodeoxyglucose (FDG)

whatwesee

FDG

FDG 6-phosphate

X

How it works: Scintillation

high energy511 keV photon

optical photons (~ 1eV)

scintillator(e.g. BGO Denseyet transparent)

currentpulse foreach UVphoton

detected

photomultipliertubes (PMTs)gain of ~ 106

Scintillators used in PET Scanners

newtechnology

veryshort

somewhatlower thanLSO

very highmoreexpensive

GSO

newtechnology

veryshort

highhighmoreexpensive

LSO

workhorselonghighestlowestexpensive

BGO

Hygroscopic

longlowesthighestcheap(relatively)

NaI(Tl)

CommentsDecaytime (µs)determinesdeadtimeandrandoms

EffectiveDensitydeterminesscannersensitivity

Effective numberof scintillationphotons @ 511keV determinesenergy and spatialresolution

CostMaterial

How it works: Timing coincidence

Δt < 10 ns?

detector A

detector B

recordpositrondecayevent

scannerFOV

β+ + e-

annihilation

Typical PET Scanner Detector Ring

Anatomy: PET gantry

Detector+ PMTassem-blies

Typical PET Image

Lung cancer example: Very obvious!

Elevated uptake of FDG (related to metabolism)

What is Attenuation?The single most important physical effect in PET imaging:• The number of detected photons is significantly reduced compared to

the number of positron decays in a spatially-dependent manner• For PET it is due to Compton scatter out of the detector ring• For CT it is a combination of Compton scatter and photoelectric

absorption

one 511 keV photonscattered out of scanner one 511 keV photon absorbed

scannerpatient

PET image withoutattenuation correction

Simulation of the Effects of not PerformingAttenuation Correction of PET Emission Image

True PET image(simulation of abdomen)

profile

Enhanced 'skin'

Reduced interior(even neg.!)

Locallyincreasedcontrast

Effects of Attenuation: Patient Study

PET: withoutattenuation correction

PET: with attenuationcorrection (accurate)

CT image (accurate)

Enhancedskin uptake

reducedmediastinal

uptake

Non-uniformliver

'hot' lungs

Attenuation Correction• Transmission scanning with an external photon source is used

for attenuation correction of the emission scan• The fraction absorbed in a transmission scan, along the same

line of response (LOR) can be used to correct the emission scandata

• The transmission scan can also be used to form a 'transmission'or 'attenuation' image

θ

sy

x

same line of response(LOR) L(s,θ)

Emission scan (EM) Transmission (TX)

tracer uptake tissue density

photon sourcerotation

t

f (x, y)

FOVscanner

PET Transmission imaging(annihilation photon imaging)

• Using 3-point coincidences, we can reject TX scatter• µ(x,y) is measured at needed value of 511 keV• near-side detectors, however, suffer from deadtime due to high

countrates, so we have to limit the source strength (particularly in 3D)

µ(x,y)

orbiting68Ge/68Ga

source

PETscanner

near-sidedetectors

511 keVannihilation

photon

And, if you have PET/CT scanner: X-ray TX

• Photon flux is very high, so very low noise• Greatly improved contrast at lower photon energies.• Scatter and beam-hardening can introduce bias.• µ(x,y,E) is measured as an weighted average from ~30-120 keV, so µ

(x,y,511keV) must be calculated, potentially introducing bias

µ(x,y)

orbiting X-ray tube and

detectorassembly

X-raydetectors

30–130 keVX-ray photon

X-ray and Annihilation Photon Transmission Imaging

Linear attenuation coeffcient at511 keV

Not a physical quantitySlowFastNoisyLow noise

PET Transmission (511 keV)X-ray (~30-120 keV)

Energy spectra

Quantitative errors in measurement

Lost (attenuated)event

Scattered coincidenceevent

Random coincidenceevent

incorrectly determined LORs

Comptonscatter

no LOR

3D versus 2D PET imaging

2D Emission Scan 3D Emission Scan

detected absorbedby septa

detecteddetected

fewer true, scattered, andrandom coincidences

more true, scattered, andrandom coincidences

septa

Effect of random coincidencecorrections in 2D and 3D

2D Emission Scan 3D Emission Scan

FOV for randomcoincidences

FOV for truecoincidences

Noise Equivalent Counts or NEC

•NEC ~ 'Effective' count rate•Prompt coincidences are what the scanner sees: P=T+S+R•but true coincidences are what we want: T=P-S-R, which addsnoise•so overall we have:

SNR2!NEC =

T

1+S/T+R/T

In this measuredexample, at 10 kBq/cc(about 6 mCi) thescanner's count rate forcoincidences will be~450 kcps, but theeffective count rate(aka NEC) will be only~75 kcps

NEC comparisons• Major arguing point for some vendors• Determined partly by detector type, detector and scanner

geometry, acquisition mode, and front-end electronics• Important, but not sole factor for image quality

0

20

40

60

80

100

120

140

0 5 10 15 20 25 30 35 40

kcps

Range for whole-body scansPeak NEC rates

Partial Volume Effect

• Apparent SUV drops with volume• Also effected by image smoothing

Final ImageFillable spheres

PET Resolution Losses

• Simulation study withtypical imaging protocols

• Limits quantitation inoncology imaging,important for followingtherapy if size changes

true tracer uptake reconstructed values(scanner resolution + smoothing of noisy data)

Time of Flight (TOF) PET/CT• Uses difference in photon detection times to

guess at tracer emission point• without timing info, emission point could be

anywhere along line• c = 3x1010cm/s, so Δt = 600 ps ~ Δd = 10 cm

in resolution

Δd = c Δt/2

timingresolutionuncertainty

best guess aboutlocation (d)

β+ + e-

annihilation

Philips Gemini TF

PET scanner LYSO : 4 x 4 x 22 mm3

28,338 crystals, 420 PMTs 70-cm bore, 18-cm axial FOV

CT scanner Brilliance 16-slice

Installation at U.Penn Nov ’05Validation and research patient imaging

Nov ’05 – Apr ’06 50 patientsBeta testing and upgrade to production release software

May ’06 – Jun ’06 40 patients (to date)

Heavy-weight patient study

13 mCi 2 hr post-inj3 min/bed

MIP

Colon cancer119 kgBMI = 46.5

non-TOF

Improvement in lesion detectability with TOF

TOF LDCT